1Field of the Invention
This invention relates to a reduced tungsten oxide electrochromic material, a method of making a reduced tungsten oxideelectrochromic material, and an electrochromic device comprising a reduced tungsten oxide electrochromic material.
2. Discussion of the Related Art
In order for reader to obtain a thorough understanding of electrochromic behavior in electrochromic materials, the reader is referred applicant's patent application entitled: "New Electrochromic Device and Method of Making an Electrochromic Layer Therefor" which was filed on Dec. 24, 1987 given U.S. Ser. No. 137,631. Since that application has a thorough discussion of electrochromic behavior in electrochromic materials, no further discussion of that subject matter will be undertaken herein.
Most simply, an electrochromic device comprises an electrochromic material layer in contact with an ion conductor material layer which is a source of ions. The electrochromic material is an insulator (dielectric material). These two layers are positioned between electrodes, at least one of which is transparent. Because the electrochromic material is a dielectric material, when a voltage is applied across the electrodes, an electric field is generated within the electrochromic material. This electric field can cause migration of the ions to the electrochromic material which produces chemical changes (electrochromic states) and corresponding color changes in the electrochromic material. The electrochromic material may change color, e.g., from colorless to blue, when it "switches" from one electrochromic state to another. By reversing the polarity of the applied voltage and hence the electric field, the electrochromic material can be switched back, e.g., from blue to colorless, that is, from its "colored" to its "bleached" state. The electrochromic material may be persistent in either its colored state or its non-colored state. By "persistent" is meant the ability of the material to remain, after removal of the electric field, in the absorptive state to which it is changed, as distinguished from a substantially instantaneous reversion to the initial state. The length of time a material is persistent is called its "open circuit memory" or simply "memory". Electrochromic devices are well known in the art and have been disclosed for example, in U.S. Pat. Nos. 3,521,941, 4,170,406, 4,194,812, 4,344,674, 4,505,538 and 4,645,308.
Literature available in this area teaches that tungsten oxide is a good electrochromic material. This electrochromic material was indicated to be fully oxidized tungsten oxide (WO.sub.3). A good electrochromic material would be understood, e.g., to exhibit a high color contrast between its colored and bleached states, be capable of being switched by a low applied voltage, display a rapid rate of coloring and bleaching, and possess a long lasting open circuit memory.
The reader is referred to Volume 823 of the proceedings of The International Society For Optical Engineering entitled: "Optical Materials Technology for Energy Efficiency and Solar Energy Conversion VI." In this volume in an article entitled: "Smart Window Coatings: Some Recent Advances", the authors on page 67 thereof give an equation for electrochromic tungsten oxide as fully oxidized tungsten oxide (WO.sub.3).
Thus it was believed that fully oxidized tungsten oxide (WO.sub.3) was a good electrochromic material. However, applicant determined that, in fact, fully oxidized tungsten oxide is not a good electrochromic material. That is, fully oxidized tungsten oxide (WO.sub.3) does not possess good electrochromic properties as defined herein. However, when applicant conducted experiments utilizing tungsten oxide in a particular oxygen deficient (i.e., reduced state), the reduced tungsten oxide was unexpectedly found to possess excellent electrochromic properties.
Applicant conducted an extensive search in the United States Patent and Trademark Office in the area of electrochromic materials and methods of making such materials. During that search, applicant did not find any prior art which was considered relevant to the subject matter disclosed and claimed herein.
It is an object of this invention to provide reduced tungsten oxide electrochromic material as a cathodic electrochromic material which has excellent electrochromic properties.
It is another object of this invention to provide a method for making such material.